Recovery of pure paraxylene



P 1953 J. D. BOOKER 2,651,665

RECOVERY OF PURE PARAXYLENE Filed June 10, 1950 SRAPED SURF/1C v CHILLER E fCENTR/FUGE '45 L IOUOR 2 0 H54 TERkL CENTRIFUGE I9; ,l 5s m LT TANK 24 HEATER *L 25 26 7 3/ I MELT r4 6 I PA RA x YLENE INVENTOR.

John D. Booker,

conducive to give best results.

Patented Sept. 8, 1953 RECOVERY OF PURE PARAXYLENE John D. Booker, Baytown, Tex., assignor, by

mesne assignments, to

ment Company, of Delaware Standard Oil Develop- Elizabeth, N. J., a corporation Application June 10, 1950, Serial No. 167,447 2 Claims. (01. 260674) The present invention is concerned with a process for separating paraxylene of over 90% purity from a mixture of it with at least one other isomeric xylene wherein the mixture contains a minor portion of paraxylene. In this particular invention the mixture is subjected to cooling to the first eutectic temperature of paraxylene and another constituent of the mixture to form a first slurry; the slurry is subjected to centrifugal action for a time suiiicient to form a first cake comprising a major portion of paraxylene and a first mother liquor. In accordance with the present invention, the first cake is heated to a temperature in the range between 30 and 40 F. to form, by partial melting, a second slurry containing paraxylene crystals in an amount in the range between 30% and 85% by weight of said second slurry. This second slurry is then centrifuged for a time suificient to form a second cake having a paraxylene content in excess of 90% and a second mother liquor, the second mother liquor being recycled or admixed at least in part with the said mixture and substantially pure paraxylene is recovered from the second cake.

The invention will be further illustrated by reference to the drawing in which the Single figure is a flow diagram of a preferred mode thereof.

Referring now to the drawing, numeral II designates a charge line through which a mixture containing paraxylene with other isomeric xylenes is introduced into the system. A typical analysis of a suitable feed stock in the practice of the present invention is as follows:

A feed stock having a composition as indicated above is discharged by line I l into a chiller, such as a scraped surface chiller, indicated by numeral I2, wherein the temperature of the feed mixture is reduced to a temperature in the range of 90 and 1l0 F. If desired, the chilled mixture may be held in a suitable holding tank or other facility, not shown, for a time, such as about 30 minutes, sufficient to allow the crystals in the slurry formed by the chilling operation to attain a form However, it may be desirable to omit the holding time since good results may be obtained without holding the cryssecond mother liquor. range from 400 to 1000 tals at the low temperature. Inany event, the slurry at a, temperature in the range between 90 and 1l0 F. is discharged from chiller l2 by line l3 and introduced thereby into a first centrifuge I4 which may be a centrifuge of the basket type. This centrifuge may operate at a rate sufiicient to impose on the slurry a centrifugal force from about 400 to 1000 times gravity. However, usually it will be desirable to employ a contrifugal force above about 500 times gravity- Satisfactory results may be obtained at centrifugal forces of about 750 times gravity. By virtue of the operation of centrifuge I4 a first cake and a first mother liquor is formed. The first mother liquor may be discharged from centrifuge I4 by line l5 for further handling as may be desired while the cake comprising substantially paraxylene crystals, with some mother liquor adhering thereto, is discharged from centrifuge [4 by line l6 which connects to branch line I! and thereby discharges the cake into a melt tank l8. Melt tank [8 is held at a sufficient temperature to cause partial melting of the crystals from the cake discharged from centrifuge I 4. It is desirable to melt from 15 to of the cake from centrifuge M to provide a second slurry containing paraxylene crystals in the range from 30% to by weight of said slurry. The second slurry formed in melt tank I8 is withdrawn therefrom by line l9 and a, portion of the second slurry i recycled to melt tank It? by line 20 containing heater 2| which introduces a sufiicient amount of heat into the slurry to allow proper functioning of melt tank [8 and also to form the second slurry. In order to insure proper control of the temperature in melt tank IS, a portion of the slurry in line 20 may be discharged directly into melt tank l8 through line 22 which will allow a portion of the slurry to by-pass heater 2|. It will be noted that line 20 connects into line I! with line l6 and allows recycling of the slurry to tank IS.

The slurry containing paraxylene crystals from melt tank I8 is discharged by line l9 into second centrifuge 23. The slurry introduced into second centrifuge 23 is at a temperature in the range between 30 and 40 F., the temperature at which the slurry is formed in melt tank [8. Centrifuge 23, like centrifuge 14, may be of the basket type and may also operate at rates sufficient to allow the formation of a second cake and a Centrifugal forces in the times gravity may also be employed in centrifuge 23.

The second mother liquor separated in centrifuge 23 is discharged therefrom by line 24 controlled by valve 25 which connects into line H. Thus the mother liquor which contains a substantial amount of paraxylene may be recycled to chiller 12 to allow substantially complete recovery of the parax'ylene in the total feed. It may be desirable-atztimes to-recycle only a portion of .the -:motheriliquor and to withdraw the unrecycled portion. Provision is, therefore, made by line 26 controlled by valve 2? to withdraw mother liquor from the system as may be desired.

The second cake formed in"centrifuge'-23 is discharged therefrom by line 28. and introduced by line 29, which connects thereto; into 'a-second melt tank 30 which is operatediatraztemperature above about 55 F. wherein the paraxylene crystals are melted to allow recoveryof liquidparaxylene from tank 30 by line 3 I. "In order to' introduce heat into the paraxylene crystals --with drawn by line 28 from centrifuge 23 a portion of the paraxylene in line 3| maybe recycled by line 32 through heater 33 for admixture with the paraxylene crystals in line 28 and for re-introduction into tank 30 by line 29.

It :will be seen from the foregoing description taken with the drawing that a simple, commercially adaptable. process is provided wherein substantially pure paraxylene of 90% by Weight and better purity may be recovered from a mixture of paraxylene containing only minor portions of The chilling temperature employed in the first stage isa function of. the composition of the feed thereto since it is desired to obtain the maximum precipitation of paraxylene crystals Without precipitation of other contaminating crystals. This isachieved by lowering the temperature to just above the eutectic point of paraxylene with the next most easily precipitatable component. As

.an example, with a feed containing 20 mole..per cent .paraxylene, 50 mole per cent A methaxylene and 30 mole per cent orthoxylene, the methaxylene in'such a concentration is soluble-at 93 F. and the orthoxylene at 90 F. Thus, by chill- .ingthe feed to about 82 F. paraxylene crystals maybe separated until 65% of the amount originally present may be recovered and the concentration of paraxylene in the residual liquor is 8 mole per cent. .For a composition of the-feed stock such as givenwabove, the temperature in the firststage will be in the range between 90 and 110 F.

.Inorder to illustratethe beneficial aspects of the present invention, 100 lbs. of a slurry containing paraxylene at about l.00 F. is introduced into a centrifuge. similar to centrifuge H.

This slurry comprises 10-lbs. of paraxylene crystals, lbs. of paraxylene liquid and 85 lbs. ofisomeric xylenes which may be termed idiluent. The equivalent of centrifuge I4 is operated at a rate sufficient to apply a centrifugal force of about 750 times of. gravity to form a mother liquor and a. cake. The mother liquor contains no paraxylene crystals, 4.9 lbs of paraxylene liquid and 83.34 lbs of diluent. On the other hand, the cake contains lbs..o1" .paraxylene crystals, .10 lb. of paraxylene liquid and 1.66 lbs. of diluent liquid for=a total of 11.76 lbs. This total of 11.76 lbs. is heatedto a temperature in the range of to 40 F. to form a second slurry which is charged to a centrifuge equivalent to second centrifuge 23 wherein a secondmother liquor correspondingto 4.7 1bs..is separated. This second mother liquor contains 3.34 lbs. of paraxylene liquid and 1.34 lbs. of diluent while .the second cake which is 4 formed in the second centifuge liquid and 0.30 lb. of diluent, making a total of 7.06 lbs. of product recovered. Therefore, this product has a purity of 96% paraxylene.

The feature of the present invention is particularly the heating of the first cake from the .first centrifugeto a temperature in the range between 30 and 40 .F. to forms. second slurry which contains from 30% to by weight of the slurry of paraxylene crystals. By virtue of the formation of the second slurry in which paraxylene crystals and paraxylene diluent are formed'itis possible to recover by centrifuging in the second stage substantially pure paraxylene while; the :recycling process allows ultimate recovery of substantial yields of pure paraxylene. .It is desirable to recycle the second mothor liquor in an amount in'the range of 20% to 250% of thepuiifred paraxylene recovered from the second centrifuge stage. A preferred range of re- ...cycle-is-from 150% to 175%. The formation of the second slurry in the first melt tank byrpar- ,tially melting the first cake is dependent on the concentration of the paraxylene remaining in the liquid recycled and that which is required for a given product purity. Stating this otherwise, the first cake is melted to that extent required to give a product having the desired purity with a given amount of recycle of the mother liquor separated from the second cake. For example, in the 20% recycle roughly 16.7% of the cake would be -melted, and in the 250% recycle, approximately 71.5% of the cake would be melted to form the second slurry.

One of the benefits of the present invention is the reduction in the time during which centrifugation takes place. Ordinarily in one stage processes in which substantially pure paraxylene is recovered, it is necessary to operate a centrifuge for long periods of time to obtain substantially pure paraxylene of over purity. In the present invention instead of operating the centrifuge in the one stage for 30 mins. or more, the total time of the centrifuging operationv is reduced to no more than 10 mins. and usually to-around 2 to 4 mins. Thus the first centrifuge is equivalent to centrifuge 14. This centrifuge would operate for no more than 10 .mins. and usually no more than 2 mins. Similar times would obtain in centrifuge 23. For example, centrifuge 23 may be operated with 2 -mi.ns. centrifuging time at 32 F. to form a cake of 93.5% paraxylene. With 2 mins. time of operation and at 40 F. a cake of purity of paraxylene would be produced. If the centrifuging time is increased to the maximum of about 10 .mins. the corresponding temperature for a given purity may be lowered to a lower temperature in the range given. As the centrifuging time is increased the temperature may be decreased.

While the present invention has been described with relationship to scraped surface chillers and basket centrifuges, it will be apparent that other chilling devices and other separation devices may be used. For example, it may be desirable to use different types of centrifuges or to use filters and other-types of chillers such as are available on the market.

The nature and objects of the present invention having been completely described and illustrated, what I wish to claim as new and useful and to secure by Letters Patent is:

1. A process for separating paraxylene of over 90% purity from a mixture of it with'at least one other isomeric xylene wherein the mixture contains a minor portion of paraxylene which comprises cooling said mixture to a temperature no lower than its eutectic point to form a first slurry of para-xylene crystals in a mother liquor, centrifuging said first slurry for no more than two minutes and sumcient to separate a first cake comprising a major portion of paraxylene and a first mother liquor, heating the first cake to a temperature in the range between --30 and 40 F. to form a second slurry containing paraxylene crystals in an amount in the range between 30% and 85% by weight of said slurry, centrifuging the second slurry for a time sufiicient to I separate a second cake having a paraxylene content in excess of 90% and a second mother liquor the time of centrifuging the second slurry plus the time for centrifuging the first slurry being no greater than a total of ten minutes, admixing at least a portion of the second mother liquor with the feed mixture, and recovering substantially pure paraxylene from said second cake.

2. A process for separating paraxylene of over 90% purity from a mixture of it with at least one other isomeric xylene wherein the mixture contains a minor portion of paraxylene which comprises cooling said mixture to a temperature in the range between 90 and -110 F. to form a first slurry of paraxylene crystals in a mother liquor, centrifuging said first slurry for no more than two minutes and sumcient to form a first cake comprising a major portion of paraxylene and a first mother liquor, heating the first cake to a temperature in the range between and F. to form a second slurry containing paraxylene crystals in an amount in the range between 30% and by weight of said slurry, centrifuging the second slurry for a time suilicient to form a second cake having a paraxylene content in excess of and a second mother liquor the time of centrifuging the second slurry plus the time for centrifuging the first slurry being no greater than a total of ten minutes, recycling the second mother liquor for admixture with said feed mixture in an amount in the range between 20% and 250% by weight based on said second cake, and recovering substantially pure paraxylene from said second cake.

JOHN D. BOOKER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,530,978 Mason Nov. 21, 1950 2,533,232 Dressler Dec. 12, 1950 2,541,682 Arnold Feb. 13, 1951 

1. A PROCESS FOR SEPARATING PARAXYLENE OF OVER 90% PURITY FROM A MIXTURE OF IT WITH AT LEAST ONE OTHER ISOMERIC XYLENE WHEREIN THE MIXTURE CONTAINS A MINOR PORTION OF PARAXYLENE WHICH COMPRISES COOLING SAID MIXTURE TO A TEMPERATURE NO LOWER THAN ITS EUTECTIC POINT TO FORM A FIRST SLURRY OF PARAXYLENE CRYSTALS IN A MOTHER LIQUOR, CENTRIFUGING SAID FIRST SLURRY FOR NO MORE THAN TWO MINUTES AND SUFFICIENT TO SEPARATE A FIRST CAKE COMPRISING A MAJOR PORTION OF PARAXYLENE AND A FIRST MOTHER LIQUOR, HEATING THE FIRST CAKE TO A TEMPERATURE IN THE RANGE BETWEEN -30* AND 40* F. TO FORM A SECOND SLURRY CONTAINING PARAXYLENE CRYSTALS IN AN AMOUNT IN THE RANGE BETWEEN 30% AND 85% BY WEIGHT OF SAID SLURRY, CENTRIFUGING THE SECOND SLURRY FOR A TIME SUFFICIENT TO SEPARATE A SECOND CAKE HAVING A PARAXYLENE CONTENT IN EXCESS OF 90% AND A SECOND MOTHER LIQUOR THE TIME OF CENTRIFUGING THE SECOND SLURRY PLUS THE TIME FOR CENTRIFUGING THE FIRST SLURRY BEING NO GREATER THAN A TOTAL OF TEN MINUTES, ADMIXING AT LEAST A PORTION OF THE SECOND MOTHER LIQUOR WITH THE FEED MIXTURE, AND RECOVERING SUBSTANTIALLY PURE PARAXYLENE FROM SAID SECOND CAKE. 